This invention relates to a pixel circuit wherein a load element disposed for each pixel is driven by current and also to a matrix apparatus wherein a plurality of pixel circuits are disposed in a matrix and particularly to an active matrix apparatus wherein an amount of current to be supplied to a load element is controlled by an insulating gate type field effect transistor provided in each pixel circuit. The present invention further relates to a display apparatus of the active matrix type which includes an electro-optical element whose luminance is controlled by a value of current such as an organic EL element as a load element.
In an image display apparatus such as, for example, a liquid crystal display apparatus, a large number of liquid crystal elements are juxtaposed in a matrix and the transmission intensity or reflection intensity of incoming light is controlled for each pixel in response to image information to be displayed thereby to display an image. Although this similarly applies to an organic EL display apparatus which uses an organic EL element for a pixel or a like apparatus, different from a liquid crystal element, an organic EL element is a self light emitting element. Therefore, the organic EL display apparatus is advantageous in that the image displayed thereon is higher in visibility than that on the liquid crystal display apparatus, that no backlight is required and that the responding speed is high. Further, the organic EL display apparatus is much different from the liquid crystal display apparatus in that the luminance level (gradation) of each light emitting element is of the current controlled type wherein it can be controlled by the value of current flowing therethrough.
For the organic EL display apparatus, two different driving methods are available including a simple matrix type driving method and an active matrix type driving method similarly to the liquid crystal display apparatus. The former has a problem that implementation of a display apparatus of a large size and a high definition is difficult although it is simple in structure. Therefore, development of organic EL display apparatus which uses the active matrix type driving method is proceeding energetically. According to the active matrix type driving method, current to flow to a light emitting element in the inside of each pixel circuit is controlled by an active element (usually a thin film transistor: TFT) provided in the pixel circuit.
Organic EL display apparatus of the type described are disclosed, for example, in Japanese Patent Laid-Open Nos. 2003-255856 and 2003-271095.
FIG. 10 shows a configuration of an exemplary organic EL display apparatus. Referring to FIG. 10, the display apparatus 100 shown includes a pixel array section 102 in which pixel circuits (PXLC) 101 are arranged in an mxn matrix, a horizontal selector (HSEL) 103, a write scanner (WSCN) 104, and a drive scanner (DSCN) 105. The display apparatus 100 further includes signal lines DTL101 to DTL10n for being selected by the horizontal selector 103 such that a signal based on luminance information is supplied thereto, scanning lines WSL101 to WSL10m for being selectively driven by the write scanner 104, and scanning lines DSL101 to DSL10m for being selectively driving by the drive scanner 105.
FIG. 11 shows an example of a configuration of a pixel circuit shown in FIG. 10. Referring to FIG. 11, the pixel circuit 101 shown is basically formed using a thin film field effect transistor (hereinafter referred to as TFT) of the p-channel type. In particular, the pixel circuit 101 includes a drive TFT 111, a switching TFT 112, a sampling TFT 115, an organic EL element 117, and a holding capacitor C111. The pixel circuit 101 formed from the elements mentioned is disclosed at an intersecting point of a signal line DTL101 with a scanning line WSL101 and a signal line DTL101. The signal line DTL101 is connected to the drain of the sampling TFT 115 while the scanning line WSL101 is connected to the gate of the sampling TFT 115, and the other scanning line DSL101 is connected to the gate of the switching TFT 112.
The drive TFT 111, switching TFT 112 and organic EL element 117 are connected in series between a power supply potential Vcc and a ground potential GND. In particular, the source of the drive TFT 111 is connected to the power supply potential Vcc, and the cathode of the organic EL element 117 (light emitting element) is connected to the ground potential GND. Since the organic EL element 117 generally has a rectifying action, it is represented by a mark of a diode. Meanwhile, the sampling TFT 115 and the holding capacitor C111 are connected to the gate of the drive TFT 111. The gate-source voltage of the drive TFT 111 is represented by Vgs.
In operation of the pixel circuit 101, the scanning line WSL101 is first placed into a selection condition (here, the low level) and a signal is applied to the signal line DTL101. Thereupon, the sampling TFT 115 is rendered conducting so that the signal is written into the holding capacitor C111. The signal potential written in the holding capacitor C111 acts as the gate potential to the drive TFT 111. Then, the scanning line WSL101 is placed into a non-selection state (here, the high level). Consequently, the signal line DTL101 and the drive TFT 111 are electrically disconnected from each other. However, the gate potential Vgs of the drive TFT 111 is held stably by the holding capacitor C111. Thereafter, the other scanning line DSL101 is placed into a selection state (here, the low level). Consequently, the switching TFT 112 is rendered conducting, and driving current flows from the power supply potential Vcc toward the ground potential GND through the TFTs 111 and 112 and light emitting element 117. Then, when the scanning line DSL101 is placed into a non-selection state, the switching TFT 112 is turned off, and the driving current does not flow any more. The switching TFT 112 is inserted in order to control the time of light emission of the light emitting element 117.
The current flowing through the TFT 111 and the light emitting element 117 has a value corresponding to the gate-source voltage Vgs of the drive TFT 111, and the light emitting element 117 continues to emit light with a luminance corresponding to the current value. Such operation of selecting the scanning line WSL101 to transmit a signal applied to the signal line DTL101 to the inside of the pixel circuit 101 is hereinafter referred to as “writing”. If writing of a signal is performed once as described above, then the light emitting element 117 continues to emit light with a fixed luminance for a period of time until writing into the organic EL element 117 is performed subsequently.
As described above, the value of current to flow to the light emitting element 117 is controlled by adjusting the voltage to be applied to the gate of the TFT 111 serving as a drive transistor in response to an input signal. At this time, since the source of the p-channel drive transistor 111 is connected to the power supply potential Vcc, the TFT 111 normally operates in a saturation region. Consequently, the drive transistor 111 serves as a current source having a current value given by the following expression (1):Ids=(½)·μ·(W/L)·Cox·(Vgs−Vth)2   (1)where Ids is the current flowing between the drain-source of the transistor which operates in a saturation region, μ the mobility, W the channel width, L the channel length, Cox the gate capacitance, and Vth the threshold value of the transistor. As apparent from the expression (1), in a saturation region of the transistor, the drain current Ids of the transistor is controlled by the gate-source voltage Vgs. Since the gate-source voltage Vgs of the drive transistor 111 shown in FIG. 11 is held fixed, the drive transistor 111 operates as a constant current source and can cause the light emitting element 117 to emit light with a fixed luminance.
FIG. 12 is a graph illustrating aged deterioration of the current-voltage (I-V) characteristic of an organic EL element. In the graph, a curve indicated by a solid line represents the characteristic in an initial state, and another curve which is indicated by a broken line represents the characteristic after aged deterioration. Usually, the I-V characteristic of an organic EL element deteriorates with time as seen from the graph. However, in the pixel circuit shown in FIG. 11, since the drive transistor is driven by constant current, the drain current Ids continues to flow through the organic EL element, and even if the I-V characteristic of the organic EL element deteriorates, the luminance of emitted light of the organic EL element does not deteriorate with time.